The present invention relates to a stripe waveguide structure type semiconductor laser device needed for optical information processing of, for example, optical fiber communications, optical measurement systems, and optical disks and a fabricating method therefor, and relates, in particular, to a fabricating method of the channel structure of a semiconductor laser device.
In this specification, the term of xe2x80x9cchannelxe2x80x9d means a path through which an electric current flows.
Conventionally, there has been a semiconductor laser that has a structure for current constriction and optical confinement on a substrate. There is a ridge structure as one of the above-mentioned structure, and there is a method for forming this ridge structure by a wet etching method.
One example of the wet etching will be described below.
First of all, as shown in FIG. 3A, for example, a GaAs layer 33 and a GaAlAs layer 32 arc etched by means of a resist mask 34. Subsequently, as shown in FIG. 3B, the GaAlAs layer 32 is selectively etched to form a GaAlAs layer 132 under the GaAs layer 33. At this time, etching depth is accurately controlled by an etching stop layer 31 under the GaAlAs layer 132.
However, according to the aforementioned wet etching method, the etched side surface cannot be prevented from having unevenness because of isotropic etching property of the wet etching method. That is, the side surface of the channel constructed of the GaAs layer 33 and the GaAlAs layer 132 comes to have unevenness. Consequently, there is a problem that, if a layer that becomes a current block layer is grown again on both sides of the channel, a recess portion on the side surface of the channel is left as a hole and crystallinity becomes significantly degraded.
If there is a cavity portion as a hole portion inside the crystals of a semiconductor laser, the subsequent processes might be limited, and complicated processes might result.
Moreover, if the crystallinity of the semiconductor laser is degraded, then adverse influence on the laser operating life and variations in characteristics result, exerting adverse influence on the yield and quality of production.
Accordingly, the object of the present invention is to provide a stripe waveguide structure type semiconductor laser device capable of preventing the occurrence of unevenness on the side surface of the channel and a fabricating method therefor.
In order to achieve the above object, there is provided a stripe waveguide structure type semiconductor laser device in which a Ga1xe2x88x92xAlxAs (0xe2x89xa6xxe2x89xa60.5) active layer, a first upper clad layer, an etching stop layer, a stripe-shaped second upper clad layer and a protective layer are successively laminated on a lower clad layer,
the second upper clad layer having different compositions in its upper portion and its lower portion, and
the etching stop layer being comprised of three different layers.
According to the stripe waveguide structure type semiconductor laser device of the above-mentioned construction, when the protective layer is formed of, for example, the GaAs layer and the second upper clad layer is formed of, for example, the AlGaAs layer, a channel is formed of a layer structure of GaAs and AlGaAs. In this case, the etching stop layer is constructed of three different layers. Therefore, etching can be performed by means of an etchant that can etch both GaAs and AlGaAs, an etchant that can selectively etch AlGaAs, an etchant that can selectively etch GaAs and an etchant that can selectively etch AlGaAs. Therefore, the unevenness of the side surface of the channel can be controlled, so that the unevenness can be prevented from occurring on the side surface of the channel. That is, an ideal channel configuration can be obtained.
Moreover, since the occurrence of the unevenness can be prevented on the side surface of the channel, even if crystals are grown on the side surface of the channel, it is enabled to prevent a hole from being left as a cavity. If a plane direction that exerts adverse influence on the laser characteristics and on operating life is known, then the plane direction can be made suppressed from appearing by adjusting the layer structure and the etching conditions.
In one embodiment of the present invention, the protective layer is comprised of a GaAs layer, and the second upper clad layer is comprised of an AlGaAs layer, and
the etching stop layer is comprised of three layers of a GaAs layer, a Ga1xe2x88x92yAlyAs (0.4xe2x89xa6yxe2x89xa60.8) layer and a GaAs layer.
In one embodiment of the present invention, the second upper clad layer has a GaAlAs multilayer structure whose Al crystal mixture ratio becomes higher in steps from the protective layer side toward the etching stop layer side.
In one embodiment of the present invention, the second upper clad layer has GaAlAs whose Al crystal mixture ratio becomes continuously higher from the protective layer side toward the etching stop layer side.
In one embodiment of the present invention, the second upper clad layer has a stripe waveguide structure formed in a forward mesa direction.
Also, there is provided a method for fabricating the stripe waveguide structure type semiconductor laser device, comprising:
a first step for etching the second upper clad layer and the protective layer by means of a first etchant that can etch both the second upper clad layer and the protective layer;
a second step for etching the second upper clad layer by means of a second etchant that possesses a great etching rate of the second upper clad layer and an impotent etching capability or a small etching rate of the protective layer and an uppermost layer of the etching stop layer;
a third step for etching the protective layer and the uppermost layer of the etching stop layer by means of a third etchant that possesses a great etching rate of the protective layer and the uppermost layer of the etching stop layer and an impotent etching capability or a small etching rate of the second upper clad layer and a central layer of the etching stop layer; and
a fourth step for etching the second upper clad layer and the central layer of the etching stop layer by means of a fourth etchant that possesses a great etching rate of the second upper clad layer and the central layer of the etching stop layer and an impotent etching capability or a small etching rate of the protective layer and a lowermost layer of the etching stop layer.
In one embodiment of the present invention, the first etchant is comprised of sulfuric acid, a hydrogen peroxide solution and water,
the second etchant is comprised of hydrofluoric solution and water,
the third etchant is comprised of ammonia, a hydrogen peroxide solution and water, and
the fourth etchant is comprised of hydrofluoric solution and water.
In one embodiment of the present invention, an etching rate achieved by the first through fourth etchants is 10 xc3x85/second to 500 xc3x85/second.